In a previous experiment, absorption of [14C]triolein was poor under low lipase in the first post-cibal hour during which luminal conditions change markedly. We wondered how low lipase might be affected by changing concentrations of fat, bile salts, titratable acid, pepsin, and food particles. Therefore, in dogs with duodenal and midintestinal fistulas, endogenous bile and pancreatic juice were excluded from the intestinal lumen and replaced with varied amounts of exogenous bile and pancreatic enzymes during steady perfusions. Oil emulsions contained [14C]triolein and [3H]glycerotriether. A double isotope ratio method and a double isotope, double extraction method were used to determine, respectively, the amount of [14C]triolein absorbed and hydrolyzed by the midgut. Lipolysis increased with both substrate and enzyme inflows, whether inflows were varied by changing concentrations or rates of volume flow. But at increasing rates of fat entry, the percent of fat hydrolyzed by the midgut declined. Neither pH 4 nor 5 citrate affected fat hydrolysis or absorption when titratable acid was infused at rates ≤16 mEq/h; but pepsin reduced both. Whereas meat particles bound lipase, their presence augmented lipolysis. We speculate that rapid gastric emptying of fat and peptic deactivation of duodenal lipase were the main factors responsible for the previously poor performance of low lipase in the first postcibal hour.